1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to techniques for efficiently determining repeatable runout (RRO) in a disk drive.
2. Description of the Prior Art
With reference to FIGS. 6A and 6B, repeatable runout (RRO) in a disk drive is a disturbance in the servo system which can degrade performance by disrupting the head's centerline tracking during read and write operations. The RRO disturbance, compared with a perfectly centered and circular data track, is typically caused by physical imperfections in the disk drive such as spindle motor runout, disk slippage, disk warping, media defects, and imperfections in the electromechanical servoing mechanism including the mechanism for writing embedded servo sectors onto the disk during manufacturing. Because the imperfections that cause RRO are relatively static, RRO is a predictable disturbance that is periodic with the rotation of the disk. It is known in the industry to estimate and cancel out the periodic RRO disturbance by introducing a feed-forward compensation signal into the servo loop.
The RRO disturbance due to the disk having a non-centric alignment with the spindle motor is sinusoidal with a period equal to the rotation of the disk. This sinusoidal disturbance can be represented as:a*cos(2πk/N)+b*sin(2πk/N)where {a,b} are coefficients corresponding to the magnitude of the disturbance (magnitude of the non-centric offset) and k is an index representing one of N servo sectors.
There is, therefore, a need for a fast, efficient technique for learning the RRO disturbance in a disk drive that may be subjected to a physical shock causing disk slippage.